This invention relates in general to the art of electronic packaging for high speed circuits, and more specifically to the packaging of electronic integrated printed circuits into assemblies, e.g. memory modules or other types of modular electronic assemblies.
Computers over time have increased in functionality and decreased in size. Driving this has been the miniaturization of circuitry and the maximum utilization of space through clever and innovative packaging techniques. Additionally, computer systems have become faster and, as a result, the need for higher speed packaging solutions has become necessary. This is particularly true for memory arrays. In computers there are two types of memory, read-only memory (ROM) and random access memory (RAM). RAM is used as a temporary work space for software applications to store data that is currently being worked on, such as documents, pictures and the like. Because of the evolution of more complex software applications, a requirement for larger and larger amounts of RAM has developed.
Computers are designed to accommodate the need for expansion of RAM by employing a packaging technique known as a xe2x80x9cmemory modulexe2x80x9d. Memory modules come in various configurations and types, and are commonly known as SIMMs (Single Inline Memory Module), DIMMs (Dual Inline Memory Module), and SODIMMs (Small Outline Dual Inline Memory Module) to name a few. Memory modules are separate electronic assemblies comprised of a plurality of memory integrated circuits and support circuitry such as line drivers, FETs or buffers, clock drivers or interface logic that is mounted on a printed wiring board. The printed wiring board has a plurality of connecting pads along one edge so that these assemblies can be installed in the computer by plugging them into connectors or sockets. A number of sockets are placed on computer motherboards so that the computer user may easily add or upgrade their RAM. The prior art has developed a foldable electronic module assembly specifically to take advantage of the assembly on which to mount extra memory components and to minimize the space consumed by the assembly. An example of this technique is found in U.S. Pat. No. 5,224,023, the ""023 patent, wherein a pair of quadrangular printed wiring boards are each laminarly split and mounted on both sides of a flexible insulating substrate having a network of conductive leads and connecting stations applied thereto. The half-sections of boards are adapted to have memory chips mounted thereon and sandwich the substrate therebetween. A pair of spaced-apart folds is made in the substrate so that it takes on an xe2x80x9cSxe2x80x9d shape that is squeezed together to save space. One of the outside half-sections of printed wiring boards has a comb of connecting terminals formed along one free edge that is plugged into a socket on the motherboard.
While this foldable electronic module assembly saves room inside the computer enclosure and allows the addition of RAM and other components to the already crowded interior of the computer system, it does not solve additional problems brought on by the demands of high performance, high speed computers systems. In the foldable module of the ""023 patent, the memory components on each successive section are located further and further distances from the interconnect point with the motherboard. As the speed of the memory data bus is increased in computer systems in order to increase the performance of the system, the successive increase in travel distance caused by the physical arrangement of the memory components of the ""023 patent induces a propagation delay that compromises or diminishes signal integrity of the memory system. It is necessary in high performance memory systems to make the trace lengths as short as possible and as equal in length as possible. The invention described below solves the trace length problems found in memory modules of the type in the ""023 patent or other types in the industry that are described as board-stacking memory modules.
In the variation of the folded module type found in U.S. Pat. No. 5,949,657 xe2x80x9cBottom or top jumpered foldable electronic assembly,xe2x80x9d and other inventions that describe high-density memory module packaging schemes of board stacking, the attachment of a second rigid printed wiring assembly to the first is accomplished by means of soldering jumper wires, connectors or pins of various types between the two assemblies.
The invention described below provides a plurality of leaves upon which are mounted integrated circuit (xe2x80x9cICxe2x80x9d) components, but which eliminates the need for additional interconnect means between the leaves because a flexible substrate itself is the means by which a mechanical and electrical connection from one component mounting leaf to another such leaf is accomplished. Furthermore the interconnect length from a signal/power terminal, e.g. a connector edge, to each leaf is achieved in as short a distance as possible. This approach significantly increases the reliability of the completed assembly by eliminating numerous solder joints and the required jumpers. In addition, the flexible substrate is preferably constructed with a ground plane on one of its layers such that each high speed signal trace has a direct return path and is spaced appropriately from the ground plane to achieve desired impedance matching characteristics required by high speed applications. Variations that incorporate jumper wires, connectors or pins do not have this signal integrity capability. In addition, a modular assembly according to this invention is maintained on a single interconnect substrate that is built and tested as one integral part, thereby reducing the assembly costs when compared to other methodologies.
While the description of the preferred embodiment below addresses assemblies of a high speed, high density memory modules, this invention is applicable as well to other types of modular electronic assemblies.
An object of this invention is to provide a means of reducing the propagation delay from a plurality of high speed, compactly packaged IC components to a computer processor or other circuit. Further objects of this invention are to provide: a means of increasing the memory capacity of memory modules; a high-speed, high density, modular electronic assembly; a packaging technique for assembling modular electronic assemblies in a compact arrangement which does not require substantial space allocation for future expansion; a packaging technique for assembling memory modules to allow the fabrication of memory modules of different capacities in the same compact package configuration such as SIMM, DIMM, SODIMM or the like; a means for efficiently upgrading existing computers by employing space-saving technology without deterioration of signal integrity between an expanded memory and a computer processor; and a xe2x80x9clow-profilexe2x80x9d memory module configuration that is high speed. A further object of this invention is to provide a high speed electronic assembly that solves multiple problems: the problem of space by providing heretofore unavailable compact electronic assemblies, and the problem of unbalanced data/control trace lengths which are caused by non-uniform lengths of signal traces inherent between successive arrays of memory components of a conventional folded electronic assembly. A further object of this invention is to provide a modular electronic assembly which accomplishes over the prior art the shortest possible trace lengths between a plurality of folded IC component leaves and a signal/power terminal by which assembly communicates with a processor or other circuit
These objects, and others unlisted above but readily discernible upon a reading of this document while viewing the appended drawings, are accomplished by a preferably pluggable electronic assembly which includes: two IC component substrate leaves; a fold coupling the two leaves together in parallel, spaced relation; a plurality of IC components distributed over and coupled to the faces of the leaves; a signal terminal projecting from one of the leaves; both leaves including signal paths for communicating signals between their respective IC components and the terminal; and the fold including signal paths for communicating signals between the leaf distal from the terminal and the terminal. Preferably the invention comprises four IC component substrates sandwiched together, the two outer substrates being rigid, the two inner substrates being separate sections of a folded flexible circuit medium; adjacent inner and outer substrates being affixed together forming two spaced leaves joined by the fold of the flexible medium with the components being distributed over and coupled to the un-affixed faces of the substrates. Preferably the folded flexible circuit medium further comprises a continuous signal reference plane. e.g. a ground plane, and power paths. Also preferably the inner substrate proximal to the terminal includes signals paths for communicating signals between its IC components and the terminal via corresponding signal paths in or on said one of the outer substrates; and both substrates distal from the terminal include signal paths for communicating signals between their respective IC components and the terminal via corresponding signal paths in or on the fold. As illustrated hereinafter the preferred embodiment includes two or more folded, separate component mounting leaves having substantially balanced signal traces, i.e., generally uniform data and uniform control signal path lengths, between the sections"" respective components and the signal terminal by which the module is signal (data and control) coupled to a processor or other circuit.
The scope of protection sought by the inventor may be gleaned from a fair reading of the claims that conclude this specification.